On the morning of the first day of each treatment period, participants will have a special echocardiogram with imaging of the heart. The echocardiogram will be performed during an infusion of dobutamine, a medicine that increases heart rate and contraction, and serves to stress the heart. This manner of stress testing is commonly used in hospitals around the country to determine if walls of the heart are receiving sufficient blood supply. That afternoon, participants will undergo a magnetic resonance imaging (MRI) study of the heart. The MRI will determine the heart's blood flow and contraction while receiving the same dosage of dobutamine as was used earlier in the day. On the morning of the second day of each treatment period, participants will exercise on a treadmill until moderately uncomfortable chest pain is reported. Later that morning, participants will undergo cardiac catheterization. For the cardiac catheterization, a long tube (catheter) will be placed into a vein of the neck once the skin is numbed with xylocaine. This tube will be positioned within the right atrium of the heart and into a tube-like structure called the coronary sinus, where venous blood exits the heart muscle. A small catheter will also be placed in an artery of the upper forearm after the skin has been numbed with xylocaine. Blood samples will be taken to allow us to measure the amount of nitric oxide transported in the blood. The blood samples will be drawn (through the tube in the heart and through the small tube in the artery) at the beginning of the study and during infusion of dobutamine to stress the heart. The dose of the dobutamine infusion will be the same dose used in the previous day's stress studies.